Absolute Extreme Ultraviolet Research Articles

Measurement of Radiated Power Loss on EAST

A type of silicon detector known as AXUV (absolute extreme ultraviolet) photodiodes is successfully used to measure the radiated power in EAST. The detector is characterized by compact structure, fast temporal response (<0.5 s) and flat spectral sensitivity in the range from ultra-violet to X-ray. Two 16-channel AXUV arrays are installed in EAST to view the whole poloidal cross-section of plasma. Based on the diagnostic system, typical radiation distributions for both limiter and divertor plasma are obtained and compared. As divertor detachment occurs, the radiation distribution in X-point region is observed to vary distinctly. The total radiation power losses in discharges with different plasma parameters are briefly analyzed.

Spectroscopic diagnostics for liquid lithium divertor studies on National Spherical Torus Experiment

The use of lithium-coated plasma facing components for plasma density control is studied in the National Spherical Torus Experiment (NSTX). A recently installed liquid lithium divertor (LLD) module has a porous molybdenum surface, separated by a stainless steel liner from a heated copper substrate. Lithium is deposited on the LLD from two evaporators. Two new spectroscopic diagnostics are installed to study the plasma surface interactions on the LLD: (1) A 20-element absolute extreme ultraviolet (AXUV) diode array with a 6 nm bandpass filter centered at 121.6 nm (the Lyman-α transition) for spatially resolved divertor recycling rate measurements in the highly reflective LLD environment, and (2) an ultraviolet-visible-near infrared R=0.67 m imaging Czerny-Turner spectrometer for spatially resolved divertor D I, Li I-II, C I-IV, Mo I, D(2), LiD, CD emission and ion temperature on and around the LLD module. The use of photometrically calibrated measurements together with atomic physics factors enables studies of recycling and impurity particle fluxes as functions of LLD temperature, ion flux, and divertor geometry.

Toroidally resolved radiation dynamics during a gas jet mitigated disruption on Alcator C-Mod

Measurements of the radiation dynamics during an Alcator C-Mod disruption induced by a high pressure He/Ar gas jet are presented. Data are analysed from four 22-channel Absolute eXtreme UltraViolet diode arrays viewing on horizontal planes from different toroidal and vertical locations, both towards and away from the gas jet. Prior to the loss of the core plasma stored energy, strong emission is seen from a shell at the plasma edge that expands toroidally and poloidally away from the the gas jet at 650 m s−1, consistent with neutral expansion rates. Both the energy loss from this region and from the bulk plasma are shown to be of comparable magnitudes and important in dissipating the electron thermal energy outside the q = 2 surface. During the thermal quench, line-integrated brightnesses exceed 100 MW m−2. But while the brightness ratio of equivalent toroidally/vertically spaced views varies from 0.4 to 4.0, energy loss integrated over the disruption is shown to be symmetric to within 25%. Radiative energy loss local to the gas jet is shown to preheat nearby surfaces making them more susceptible to melting when the bulk of the thermal energy is lost.

Two dimensional radiated power diagnostics on Alcator C-Mod

The radiated power diagnostics for the Alcator C-Mod tokamak have been upgraded to measure two dimensional structure of the photon emissivity profile in order to investigate poloidal asymmetries in the core radiation. Commonly utilized unbiased absolute extreme ultraviolet (AXUV) diode arrays view the plasma along five different horizontal planes. The layout of the diagnostic set is shown and the results from calibrations and recent experiments are discussed. Data showing a significant, 30%-40%, inboard/outboard emissivity asymmetry during ELM-free H-mode are presented. The ability to use AXUV diode arrays to measure absolute radiated power is explored by comparing diode and resistive bolometer-based emissivity profiles for highly radiative L-mode plasmas seeded with argon. Emissivity profiles match in the core but disagree radially outward resulting in an underprediction of P(rad) of nearly 50% by the diodes compared to P(rad) determined using resistive bolometers.

Soft and Ultra-Soft X-ray Detector Array Systems for Measurement of Edge MHD Modes in the Large Helical Device

In the Large Helical Device (LHD), several 20-channel Soft X-ray (SX) detector arrays are used to observe the radial structures of SX fluctuations related to MHD instabilities. Recently, 20-channel absolute extreme ultraviolet (AXUV) detector arrays have also been installed inside the vacuum vessel in the vertically and horizontally elongated sections of LHD to monitor MHD fluctuations of AXUV emissions emitted from edge plasma region. These AXUV detector arrays have often detected edge MHD fluctuations localized in low temperature and density edge region so that the SX detector arrays cannot be detected. In LHD, these SX and AXUV detector array systems are successfully employed in order to investigate radial structure and temporal behaviors of edge MHD modes. c © 2007 The Japan Society of Plasma Science and Nuclear Fusion Research

Determination of the Major Impurity Radiators in the Reheat Mode Discharges in the Compact Helical System

Radiation brightness and impurity behaviors have been studied for reheat mode discharges in the Compact Helical System (CHS) by three different types of impurity diagnostics. Total radiation power measured by a pyroelectric detector significantly reduces after entering the reheat mode, whereas the line-averaged radiation brightness measured by an absolute extreme ultraviolet (AXUV) photodiode array increases especially for a center viewing chord due to the impurity accumulation in the plasma core. One possible reason for this opposite behavior between the two bolometric detectors is the reduced sensitivity of the AXUV photodiode for lower energy photons in vacuum ultraviolet (VUV) region. This speculation is supported by temporal evolutions of VUV spectra measured by a grazing incidence spectrometer. These results demonstrate that the comparison of three impurity diagnostics would be beneficial to the determination of the major impurity radiators and a comprehensive understanding of impurity behaviors in the reheat mode discharges.

Application of Tomographic Imaging to Multi-pixel Bolometric Measurements

An improved tomographic algorithm in the scheme of Tikhonov-Phillips regularisation method, has been employed for multi-pixel bolometric measurements in order to get as much information as possible while keeping the assumptions to a minimum. The effects of finite detector size have been taken into account with a full three dimensional treatment of the detector geometry. The application of tomographic imaging was implemented to a two-array AXUVD (Absolute eXtreme UltraViolet photodiode) camera on the Large Helical Device (LHD) and a two-dimensional infrared imaging bolometric (IRVB) pinhole camera on JT-60U. Pertinent examples of the results are presented both to illustrate the analysis techniques and to demonstrate the wealth of physics which can be studied.

Characteristics of Radiating Collapse at the Density Limit in the Large Helical Device

Steady state densities of up to 1.6 × 1020 m-3 have been sustained using gas puff fuelling and NBI heating up to 11 MW in the Large Helical Device (LHD). The density limit in LHD is observed to be greater than 1.6 times the Sudo limit. The density is ultimately limited by a radiating collapse which is attributed to the onset of a radiative thermal instability of the light impurities in the edge region of the plasma based on several recent observations in LHD. First of all the onset of the radiative thermal instability is tied to a certain edge temperature threshold. Secondly, the onset of the thermal instability occurs first in oxygen and then carbon as expected from their cooling rate temperature dependencies. Finally, radiation profiles show that as the temperature drops and the plasma collapses the radiating zone broadens and moves inward. In addition, comparison of impurity lines with the total radiated power behaviour suggests that carbon is the dominant radiator. Two dimensional tomographic inversions of Absolute eXtreme UltraViolet Diode (AXUVD) array data and comparison of modelling with images of radiation brightness from imaging bolometers indicate that the poloidal asymmetry which accompanies the radiating collapse is roughly toroidally symmetric.

Plasma radiometry with 30 chord resolution for fast transients in the DIII-D tokamak

A diagnostic capable of providing time resolved measurements of plasma radiated power during disruptions and other fast transients, e.g., edge localized modes has been employed in the DIII-D tokamak. The radiation is detected with absolute extreme ultraviolet (AXUV) photodiode arrays. Thirty chords from a single port provide measurements from a full slice of the plasma at one toroidal location. The analog bandwidth is up to 1 MHz for the brightest events, i.e., disruptions. Active cooling of the diode arrays prevents damage during high temperature vessel baking. Effective responsivity values of 0.12–0.18 A/W are taken from previous work on the application of AXUV diodes in DIII-D. The total radiated energy in disruptions typically agrees with bolometer measurements within about 12%.

Time resolved radiated power during tokamak disruptions and spectral averaging of AXUV photodiode response in DIII-D

Silicon absolute extreme ultraviolet (AXUV) photodiodes have been employed in a disruption radiometer diagnostic for measurement of radiant power in the DIII-D tokamak with a 170 kHz bandwidth. This is motivated by a need to improve the understanding of radiative processes in tokamak disruptions. The diagnostic described in this article has a single line of sight though the central plasma. Accounting for the photon energy dependence of the AXUV photodiode responsivity is made possible by optical filtering, with the aid of spectra from an extreme ultraviolet survey spectrometer. The appropriate effective responsivity for interpretation of the data is lower than the nominal value typically used for the detector. In the current quench phase of disruptions, it is less than half the nominal value. Comparisons with results from a foil bolometer find good agreement.

Local Impurity Deposition in a Magnetic Island by Means of a Tracer-Encapsulated Solid Pellet in the LHD.

Local impurity deposition in the n/m = 1/1 magnetic island, which is expanded by the external perturbation coils, has been obtained by means of the tracer-encapsulated solid pellet in the Large Helical Device. The differences of the local deposition of the tracer between inside the magnetic island and outside that were clearly observed by both the absolute extreme ultra violet silicon photodiode arrays and the ECE radiometer.